It is important to recognize that bonding between Mg and boron ring is unusual, you should not expect that BCP must occur between every pair of Mg-B.
In my opinion, BCP analysis is never suitable for charactering bonding between Mg and boron ring of this system, I highly suggest studying Mg-B interactions by means of other methods, for example, plotting ELF (or LOL or deformation density), plotting density difference between the boron ring and Mg2, examining fuzzy or Mayer bond order, examining atomic charges and so on
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